Method of rinsing conversion coatings with chromium complex solutions from chromic acid



. spraying or dipping.

United States Patent "ice METHOD OF RINSING CONVERSION COATINGS WITHCHROMIUM COMPLEX SOLUTIONS FROM CHROMIC ACID James I. Maurer, St. ClairShores, Richard E. Palmer,

Farmington, and Vinod D. Shah, Detroit, Mich., assignors to HookerChemical Corporation, New York, N.Y., a corporation of New York NoDrawing. Filed Oct. 15, 1962, Ser. No. 230,729

The portion of the term of the patent subsequent to Dec. 7, 1982, hasbeen disclaimed 2 Claims. (Cl. 1486.16)

This invention relates to improvements in metal coating and moreparticularly relates to solutions for and a method of improvingchemically formed coatings on metal surfaces as a base for paint,laquer, varnish or other organic finishes.

In the art of preparing metal surfaces to receive paint, it is nowwidely understood that the application of a dilute chromic acid rinse tophosphate coatings and oxide coatings on the metal surface improves thatcoating as a base for paint, usually both with respect to corrosionresistance and humidity resistance of the painted surface. The use ofsuch rinses is now quite conventional and widely used. Such use is notwithout difficulty, however, and one of the long standing problems hasbeen the avoidance of uneven concentrations of the chromic acid ondifferent areas of a rinsed metal surface. Such uneven concentrationsproduce blistering, peeling or other premature failure of the paintapplied thereover. A number of solutions have been proposed to overcomethis problem including the use of specially controlled and formulatedrinses subsequent to the conventional chromic acid rinse, theapplication of the dilute chromic acid rinse by atomization on a hotbare or chemically coated surface, and the use of dried on coatingscontaining hexavalent chromium compounds, reduced hexavalent chromiumcompounds or mixtures thereof. While these proposals represent advancesin this art and have utility in specific applications, they each havesubstantial drawbacks from the commercial standpoint. There is still animportant need for an inexpensive, simple process which is adaptable foruse in conventional apparatus in high speed production line operation toimprove the corrosion and humidity resistance of conversion coatings onmetal surfaces.

In commercial high speed production line processing of metal parts forpainting, the predominant processes apply the phosphate or otherchemical coating from an aqueous bath and after water rinsing the formedchemical coating, it is subjected to an aqueous chromic acid rinse,either by In many of these processes, the chromic acid rinsed coating isthereafter water-rinsed to remove uneven concentrations of chromic acidon certain areas of the surface. It is Well known that the final waterrinse dissolves a substantial portion of the applied chromic acid rinseand yet the final water rinse continues to be employed becausenon-uniform distribution of chromic acid on the surface causes moreserious paint failure than ,results from the reduction in the chromicacid content in the final rinsing step. Moreover, the final water rinsehas the advantage that it prepares the surface for use with all types ofpaints and painting systems, even including those which are susceptibleto contamination by excess chromic acid in the coating, since the rinseeliminates any water soluble chromic acid.

This invention provides a process which includes the conventional stepswhich are predominantly employed in high speed production lineoperation, namely, conventional cleaning, phosphate or other conversioncoating forming steps and thereafter rinses the phosphate or conversioncoating with the modified aqueous rinse solution of this 3,279,958Patented Oct. 18, 1966 invention, and finally employs the step of waterrinsing the rinsed conversion coating to insure the absence of unevenconcentrations of rinse material on the surface.

This invention is based on the discovery that certainchromium-containing solutions are capable of forming water-insolublechromium-containing complexes with certain integral chemical coatings ona metal surface by a simple rinsing step in a short time, whichcomplexes remain on and in the coating during a subsequent water rinsingstep. It was also found that the application of such rinse solutions,under the herein below specified conditions, forms such water-insolublechromium-containing complexes in unexpectedly large quantities as anintegral part of the conversion coating. The evidence at hand clearlydemonstrates that coatings such as phosphates, mixed phosphate-oxides,chromates, oxalates and arsenates on the surfaces of metals which aresusceptible to corrosion such as iron, steel, zinc, aluminum and thelike, after painting, show substantially better corrosion resistance,humidity resistance or paint adhesion, when they are rinsed in thesolutions of this invention than when they are rinsed in the heretoforeused dilute chromic acid solutions.

In accordance with this invention, it has been found that in order tocontrol the reaction or interaction between the chemical conversioncoating and the applied rinse solution which produces the waterinsoluble chromium complex in that coating the rinse solution should bea dilute aqueous solution containing chromium complexes which include ananion portion that is displaceable upon contact by an anion in thechemical coating. When such a chromium complex contacts the chemicalcoating, the complex is modified to contain the anion of the surfacecoating, and the chromium complex thus becomes attached to and anintegral part of the coating. It has been found that in order to formdilute aqueous solutions which contain such a chromium complex, it isnecessary to provide a solution containing at least about 0.001% w./v.of the trivalent chromium ion and having a pH within the range of about3.8 to about 6.0. As used herein and in the appended claims,concentrations are expressed as percent w./v. or percent weight per unitvolume of the solution, and pH refers tot he numerical values obtainedfrom solutions prepared with deionized water. The pH should be adjustedin the rinse solution prior to the application of the solution to thesurface since the necessary chromium complex must be present in therinse solution at the moment of contact of the chemical coating.

The chromium complex which becomes attached to the chemical coating iswater insoluble and the entire complex remains in place through thesubsequent water rinse step. When using the rinse solution of thisinvention it has been found that there is a substantial increase in thechromium concentration in the coating after water rinsing relative tothat which is present in the coating after the use of the heretoforeknown dilute chromic acid solutions. It has also been found that theimprovement in corrosion and humidity resistance after painting which isobtained correlates with the increase in the integral water insolublechromium complex which is found in that coating. However, in order toform the needed chromium complexes in the solution to enable thisinteraction with the coating on the surface it has been found that thepH of the rinse solution containing at least about 0.001% tri-valentchromium ion must be adjusted to within the range of about 3.8 to 6.0.When the pH of the rinse solution is below about 3.8, the tendency forthe formation of the necessary complexes in the rinse solution isreduced and the 'use of such solutions produces no commerciallysignifimodified dilute chromic acid rinse solutions. When the pH of therinse solution of this invention exceeds about 3.8, however, it ispossible to detect a significant increase in the concentration ofchromium which remains on the surface in a water insoluble form relativeto that which remains on a similar surf-ace treated with a conventionaldilute chromic acid rinse solution containing an identical concentrationof chromium, and the corrosion resistance to salt spray is measurablyimproved relative to the conventionally rinsed coatings. When the pH ofthe rinse solution of this invention exceeds about 6, there is atendency for the complex to become insoluble. It has been observed,however, that the solubility of the complex varies not only with thenumerical pH value but is also a function of the additional anions whichare present in the solution, for example, oxalate or the like, andgelation or precipitation does not always occur precisely at a pH of 6.The upper limit of pH referred to in the claims as about 6 means thatvalue at which the chromium complex remains stable in the dilute aqueoussolution.

As above indicated, the dilute aqueous rinse solutions of this inventionshould contain chromium complexes which include an anion thatis'displaceable by the anion portion of the coating to be rinsed. Whilethe most commercially important coating of this type is a phosphatecoating, it is to be understood that the coating may be of thephosphate-oxide type such as is obtained from the use of alkali metaldihydrogen phosphate solutions, e.g., sodiu-m or potassium or ammoniumdihydrogen phosphate solutions, or may be a chromate coating, an oxalatecoating, or an arsenate coating. The method is adaptable for use inconjunction with such chemical coatings on the surfaces of any metalwhich is susceptible to corrosion in the atmosphere or under othercorrosive conditions and particularly including iron, steel, zinc,aluminum, copper, brass, bronze, magnesium, titanium and the like. Ithas been found that the anion of phosphate coatings most effectivelydisplaces from a chromium complex one or more of the anions nitrate,chloride or chromate. The P0, anion will displace other anions to asomewhat lesser degree, such as sulfate ion, the formate ion and theacetate ion. When the coating is other than phosphate, for example, onewhich contains the oxalate anion, the dilute rinse solution containingthe chromium complex may satisfactorily be one which contains any one ofthe anions nitrate, chloride, sulfate, phosphate, formate, acetate orsulfite, and when the coating is chromate or arsenate, the preferredanions are nitrate and chloride or mixtures thereof. When rinsing any ofthe above specified surface coatings, the dilute chromium complex rinsesolution may contain certain other anions as well as those abovementioned, such as the nitrite, phosphate, chromate, oxalate or chlorateanions, and it is to be understood that in any case the rinse solutionmay contain chromium complexes which are mixtures of complexescontaining one or more of the above named anions. The presence of suchother anions does not prevent the interaction between the chromiumcomplex and the coating and in the case of the chromate anion theconcentration may even exceed the chromium complex concentration and yetthe benefits of the invention are still obtained, although the degree ofinsoluble complex formation in the coating does vary as theconcentration of the other anion is increased, or decreased, andordinarily only relatively small amounts of such other anions should bepresent.

Therinse solutions of this invention may contain trivalent chromium inconcentrations within the range of about 0.001% w./v., up to the limitof solubility of the selected chromic salt. A preferred operatingconcentration is within the range of 0.01% to 0.25% and for typicalcommercial phosphate coating rinse applications no advantage has beenfound from the use of trivalent chromium concentrations exceedingabout0.1% w./v.

The rinse solutions of this invention are those which are prepared fromaqueous chromic acid solutions by reducing the chromic acid with any ofa large number of known materials which are capable of reducing thehexavalent chromium ion to the trivalent state. Materials which containan active hydroxyl, aldehyde or carboxyl group are suitable for thispurpose and the materials of this type which are capable of relativelyfast reduction of the hexavalent chromium ion are preferred. Suitablerinse solutions are formed so long as the quantity of reduction isinsufficient to form a gel.

Examples of such materials include the mono hydroxy alcohols includingmethyl, ethyl, propyl, isopropyl and butyl alcohols, etc., the dihydroxyalcohols, such as glycol, polyethylene glycols, and the polyhydroxyalcohols such as glycerine, mannitol, sorbitol; the aldehydes includingaliphatic and aromatic aldehydessuch as formaldehyde, acetaldehyde,be'nzaldehyde; phenol and carboxylic acids including citric acid,tartaric acid, etc. The reducing agent is preferably one which iscapable of causing reduction of the hexavalent chromium ion to thetrivalent chromium ion in a relatively short time.

Excellent results have been obtained from the use of the productobtained by reducing chromic acid with methyl alcohol in an amount inthe range of about 1% to about 60% reduction. The procedure employed forproducing the desirable reduction product includes the steps of slowlyadding an aqueous methyl alcohol solution to an aqueous chromic acidsolution, and after preliminarily mixing, the mixture is furtherreacted, with agitation, preferably at a temperature in the range ofabout F. to about F. for about 5-36 hours, depending upon the timerequired to complete the reduction for the quantity of methyl alcoholpresent. As the temperature is increased, the speed of reduction or rateof the reaction similarly increases. The specific degree of reductionhas not been found to be critical and good results have been obtained inrinsing zinc phosphate coatings or phosphate-oxide coatings prior topainting with rinse solutions having between about 1% and about 55%reduction of the hexavalent ion to the trivalent ion.

In solutions within \a range of pH of about 3. 8 to about 6.0, theformation of the needed chromium complexes occurs fairly rapidly so thatit is unnecessary to age the rinse solution for an extended period oftime prior to use. For example, a solution prepared at normal roomtemperature and adjusted to within the above stated pH range will haveformed sufficient r rorni-um complexes to be effective as a rinse in theinterval of time which is required to confirm that the pH is within thedesired range.

The method of this invention simply comprises the application of thedilute rinse solution to the preliminarily formed chemical coating onthe metal surface to be prepared to receive paint or other organicfinish in a conventional manner such as by spraying, dipping, brushingor the like. After the rinse solution is permitted to drain from thetreated chemically coated metal surface, the surface is subjected to awater rinse to insure the absence from uneven concentrations ofprimarily water-soluble chromium in land on the coating and afterdrying, the surface is ready to receive an organic finish coating. Theafter rinse solution is preferably deionized water and excellent resultsare obtained when it is applied in accordance with the method disclosedin Richards United States Patent No. 3,304,933. The benefits of theinvention are obtained even though the final rinse is tap water or otherwater which is not contaminated with unusually high concentrations ofundesirable anions such as chlorides, sulfates, etc. After the finalwater rinse the coating may be permitted to dry in air or, if desired,may be dried in an oven or with forced Where greater speed is necessary.

It has been found that when using the rinse solutions of this inventionon a continuous basis that the rinse solutions tend to become 1 oreacid, and if the solutions are not adjusted periodical-1y by theaddition of alkaline material, the pH of the rinse solution will migratebelow 3.8. It is necessary to check the pH of the solution during useand in order to obtain the improvements which characterize thisinvention to adjust the pH so that it is Within the lherein claimedrange. It has also been observed that the chromium complex rinsesolutions of this invention have the same tendency upon mere standingand if substantial periods of time occur subsequent to the preparationof a rinse solution it is important to check the pH of that rinsesolution before it is used, and if necessary, to adjust it within theclaimed nange.

The fiollowing examples illustrate the invention in somewhat greaterdetail but it is to be understood that the specific compositions,conditions of treatment and products produced are given for purposes ofi-llustnation only and are not intended to set forth the definitivelimits of the invention which have been given hereinabove.

Example I An aqueous acidic zinc phosphate coating solution, modifiedfor coating zinc, was prepared in a conventional manner and uponanalysis found to contain 0.27% zinc, 0.23% nickel, 0.98% P 0.21%fluoride, added as silicofluoride, 0.2% N0 and having a total acid of 26points.

A number of 4" x 12" continuous hot dipped galvanized panels werecleaned in a conventional titanated cleaner, and phosphate coated byspraying the above solutions, at 160 F on the surfaces for 1 minute andtherafter cold water rinsing for 30 seconds. A number of these phosphatecoated panels were then rinsed in a dilute aqueous chromic acid solutioncontaining 0.05% CrO having a pH of approximately 3.5 fior 30 seconds at125 F., then rinsed in deionized water by spraying for 5 seconds at roomtemperature and dried.

A dilute trivalent chromium containing solution was prepared by slowlyadding 2,010 ml. of 20%, by volume, aqueous methyl alcohol solution to51 lbs. of 25.5% by weight CrO aqueous solution. The rate of alcoholaddition was controlled to insure that the temperature in the reactionsolution did not exceed 175 F. and thereafter the total volume wasincreased to gallons by adding additional water. The dilute material wasthen heated for 6 hours, with stirring, at 170 F.l75 F. and at the endof this reaction period water was added to form a total of 93 lbs.reduced chromic acid concentrate. A portion of this concentrate wastested and found to have a CrO content of 9.6% by weight. When deionizedwater was added to a portion of the concentrate to produce aconcentration of 0.016% trivalent chromium, the pH was found to be 3.3.A port-ion of the 3.3 pH solution was adjusted by adding sodiumhydroxide solution thereto to form a solution having a pH of 4.6.

A number of the panels coated with zinc phosphate, as above described,were rinsed in the 4.6 pH rinse solution by spraying for seconds at 125F., and then rinse-d in deionized water by spraying for 5 seconds atroom temperature and dried.

All of the panels were then coated with paint using the commercialtwo-coat enamel, Dulux 7076741, and subjected to the standard 5% sodiumchloride salt spray test.

After 672 hours, an inspection of the panels rinsed in the chromic acidsolution having a pH of 3.5 showed that the panels were corroded anaverage of A" to /2 from the diagonal scratch marks and containedlocalized spots along the scratch ma-rlcs having diameters of aboutAfter 672 hours, the panels which were rinsed in the methyl alcoholreduced chromic acid solution having a pH of 4.6, upon inspection, werefound to have no general cree-page from the scratch marks and to haveonly a few spots along the scratch marlcs having diameters of aboutExample 11 An aqueous acidic chroma-ting solution for coating aluminumwas prepared and upon analysis found to contain 0.25% CrO 0.35%fluoride, added as HP, 0.1% aluminum, 0.08% K Fe (CN) and having a pH of1.6. A number of 4" x 12" 3003 type aluminum alloy panels wereconventionally cleaned in a non-etching cleaner and ehromatecoated withthe above described solution by spraying the solution on the panels for15-20 seconds at F. The coating produced was a mixed oxidechromatecoating having an average weight of about 3040 rnillignams per squareft. A portion of the coated panels were then rinsed in a dilute chromicacid rinse solution containing 0.09% CrO and aving a pH of approximately3.5 by immersing the panels in the solution for 30 seconds at F., andthereafter the panels were rinsed in deionized water by flushing thesurface and the panels were then dried in an oven at 200 F. for 2minutes.

Another series of the chromate-coated panels were rinsed in the reducedchromic acid rinse solution described in detail above in Example I, byimmersion for 30 seconds at 125 F., and thereafter rinsed in deionizedWater by flushing and thereafter dried in an oven at 220 F. for 2minutes.

All of the panels were then painted with the two-coat enamel, Dulux707-6741, and subjected to the scratch adhesion test. The panels whichwere rinsed in the chromic acid rinse solution having a pH of 3.5 had anadhesion rating of 7-8, whereas the panels rinsed in the reduced chromicacid rinse solution having a pH of 4.6 had an adhesion rating of 8-9.The adhesion test rates the adhesion of the paint to the surface, thetest comprising an attempt to scrape the paint from the surface by aknife-blade applied thereto at constant angle and pressure. The numeral10 represents excellent adhesion, 8, good adhesion, 6, fair adhesion,etc.

Example III A dilute trivalent chromium containing solution was preparedby slowly adding 28 grams of a 25% aqueous solution of formaldehyde to364.4 grams of a 25% CrO by weight aqueous deionized water solution. Thehighest temperature reached during mixing was 174 F. and after partialcooling the reaction was continued at F. for 2 /2 hours and adetermination showed that the reaction had elfected a 27% reduction ofthe CrO to trivalent chromium. A portion of the reduced concentrate wasdiluted with ordinary tap water to produce a solution containing 0.0138%trivalent chromium and having a pH of 3.9.

A number of 4" x 12" cold rolled steel panels were coated with a zincphosphate coating by using a solution containing 0.23% zinc, 0.45% P00.56% calcium, 2.48% nitrate, 0.17% ferrous iron, having a total acid of15.1 and a free acid number of 1.0, by spraying for 60 seconds at 177 F.followed by a 30-second cold water rinse. A series of such coated panelswere then rinsed in a dilute aqueous chromic acid rinse solutioncontaining 0.05% CrO and having a pH of approximately 3.5 by immersionat room temperature for 30 seconds. After withdrawal from the chromicacid rinse solution the panels were sprayed for 10 seconds withdeionized water and dried for 3 minutes at 375 F.

Another series of similarly phosphate coated panels were rinsed in thetrivalent chromium containing solution having a pH of 3.9 by immersionat room temperature for 30 seconds, followed by a 10-second spray withdeisonized water and drying for 3 minutes in an oven at 37 P.

All of the panels were then coated with paint using the commercialtwo-coat enamel system, Dulux 7076741, and subjected to the standardsodium chloride salt corrosion test.

After 792 hours an inspection of the panels rinsed in the chromic acidsolution having a pH of 3.5 showed that the migration of the corrosionfrom the diagonal scratch marks was between and /2 and that the 7 panelshad from 10-12 spots of corrosion on their surfaces. After 792 hours thepanels which were rinsed in the formaldehyde reduced chromic acidsolution having a pH of 3.9, upon inspection, were found to havemigration of the corrosion from the diagonal scratch marks of zero andMe" and to have 3 spots of corrosion on their surfaces.

Example IV A dilute trivalent chromium containing solution was preparedby slowly adding grams of benzaldehyde to 404.3 grams of a 25% byweight, CrO -deionized water solution. There was no discernible rise intemperature which resulted from the mixing. The admixture was thenheated to 160 F.170 F. and maintained at that temperature for 18-20hours. At the end of this time, analysis of the product revealed thatapproximately 3.5% of the CrO had been reduced to trivalent chromium. Aportion of this reduced concentrate was diluted with ordinary tap waterto form a solution containing 0.001236% trivalent chromium.

A number of 4" x 12" cold rolled steel panels were coated with a zincphosphate coating by using the solution and procedures described abovein detail in Example III. A series of these coated panels were rinsed ina dilute chromium aqueous rinse solution containing 0.05% CrO and havinga pH of approximately 3.5 by immersion at room temperature for 30seconds, and thereafter deionized-water rinsing for seconds by sprayingat room temperature and then drying the panels for 3 minutes at 375 F.

Another series of the same phosphate coated panels were rinsed in thetrivalent chromium containing solution, at a pH of 5.0, by immersion atroom temperature for 30 seconds, spray rinsed in a deionized water sprayfor 10 seconds at room temperature, and thereafter dried for 3 minutesin an open at 375 P.

All of the panels were then coated with paint, using the commercialtwo-coat enamel system, Dulux 707-6741, and subjected to the standardsodium chloride salt corrosion test.

After 792 hours, an inspection of the panels rinsed in the chromic acidrinse solution having a pH of 3.5 showed that the migration of thecorrosion from the diagonal scratch marks was between and /2" and thatthe panels had from 10-12 spots of corrosion on their Surfaces. After792 hours the panels which were rinsed inthe benzaldehyde chromic acidsolution having a pH of 5.0, upon inspection, were found to have amigration of the corrosion from the diagonal scratch marks of betweenand and to have 2-3 spots of corrosion on their surfaces.

What is claimed is:

1. A method of preparing integral chemical coatings, selected from thegroup consisting of phosphate coatings, phosphate-oxide coatings,chromate coatings, oxalate coatings and arsenate coatings, on a metalsurface to receive an organic finish which comprises the steps ofapplying to said chemical coating a dilute aqueous solution containingat least about 0.001% trivalent chromium in the form of a chromiumchromate complex, said solution having a pH in the range of about 3.8 toabout 6, and thereafter water rinsing said surface.

2. A method of preparing integral chemical coatings, selected from thegroup consisting of phosphate coatings, phosphate-oxide coatings,chromate coatings, oxalate coatings and arsenate coatings, on a metalsurface to receive an organic finish which comprises the steps ofapplying to said chemical coating a dilute aqueous solution containingat least about 0.01% to about 0.25% trivalent chromium in the form of achromium chromate complex, said solution having a pH in the range ofabout 3.8 to about 6, and thereafter water rinsing said surface.

References Cited by the Examiner UNITED STATES PATENTS 900,597 10/1908Salzer 20451 1,922,853 8/ 1933 Kissel. 2,768,104 10/1956 Schuster et al.1486.16 2,846,342 8/1958 Curtin 148-616 2,882,189 4/1959 Russell et a11486.16 2,911,332 11/1959 Schuster et a1. 1486.2 3,063,877 11/ 1962Schilfman 1486.2 X 3,094,441 6/ 1963 Curtin 148-616 OTHER REFERENCESMellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry,Longmans Green Co., 1931, vol. 11, p. 235.

ALFRED L. LEAVITT, Primary Examiner.

WILLIAM D. MARTIN, RICHARD D. NEVIUS,

R. S. KENDALL, Assistant Examiners.

1. A METHOD OF PREPARING INTEGRAL CHEMICAL COATINGS, SELECTED FROM THEGROUP CONSISTING OF PHOSPHATE COATINGS, PHOSPHATE-OXIDE COATINGS,CHROMATE COATINGS, OXALATE COATINGS AND ARSENATE COATINGS, ON A METALSURFACE TO RECEIVE AN ORGANIC FINISH WHICH COMPRISES THE STEPS OFAPPLYING TO SAID CHEMICAL COATING A DILUTE AQUEOUS SOLUTION CONTAININGAT LEAST ABOUT 0.001% TRIVALENT CHROMIUM IN THE FORM OF A CHROMIUMCHROMATE COMPLEX, SAID SOLUTION HAVING A PH IN THE RAGE OF AOUT 3.8 TOABOUT 6, AND THEREAFTER WATER RINSING SAID SURFACE.